Calcification of glutaraldehyde-preserved or “fixed” bioprosthetic heart valves frequently occurs and leads to failure due to stenosis and regurgitation. In addition, the slow release of glutaraldehyde from the implanted device is cytotoxic. Several methods of tissue cross linking or fixing that are independent of glutaraldehyde have been presented, and they include acyl azide, photooxidation, epoxy, genipin and carbodiimide. The latter is described in U.S. Pat. No. 5,733,339 issued Mar. 31, 1998. However, it is acknowledged that tissue shrinkage occurs during fixation, and no cross linking method has yet been perfected that totally avoids tissue shrinkage (J. Heart Valve Dis. 2001; 10(1): 111-124). While this may be of no particular concern with respect to pericardium or tissue for which tailoring will take place after fixation, it may well be an issue for porcine aortic root leaflets where precise interengagement, i.e. coaptation, is very important and excessive shrinkage may induce insufficient coaptation of the cusps which could render the valve incompetent. Therefore, the need for improved fixation technology that induces only minimum tissue shrinkage still exists, and the search for such technology has continued.
In addition to the problem of tissue shrinkage, it has also been found that various prosthetic devices require different degrees of cross linking. In general, cross linking provides the advantage of blocking or masking antigenic moieties within the protein structure of the prosthesis, thereby reducing or eliminating the body's immune response to the foreign object. Cross linking can also increase the durability of the prosthetic device by rendering the surface of the prosthesis resistant to biochemical degradation. Finally, cross linking can impart some rigidity to the prosthesis, which is often desirable for particular applications.
In contrast, a less highly cross linked tissue that is used as a prosthetic device will be expected to possess less structural rigidity, and thus to be more flexible. In addition, it would be expected that such tissues would be more likely to induce an immune response and to biodegrade over time. A tissue with little or no cross linking would be expected to exhibit properties similar to those of fresh tissue. It would be expected to be relatively fragile, flexible and susceptible to biodegradation. In addition, unless steps are taken to block antigenic sites that are normally blocked during the cross linking procedure, a non-cross linked tissue would also be expected to elicit immunological responses similar to those elicited by fresh tissue.
While fully cross linked tissue possesses many advantages, there are applications in which minimally cross linked tissue and partially cross linked tissue would make a superior implant. A partially cross linked tissue would be useful in applications where flexibility of the cross linked tissue is of primary importance. Thus, a minimally cross linked tissue would be useful in applications where maximum flexibility would be desirable; e.g. in subdermal reconstructive surgery, etc.
Thus, there is a need for a process of preparing a bioimplant in which the degree of cross linking can be varied from minimally to partially to fully cross linked. There is similarly a need for a such a process in which the partially and minimally cross linked tissues do not elicit an immune response. These and related needs are satisfied by embodiments of the present invention.